The problem of measurement of thin coatings of metals on metals has been resolved over several decades by using fluorescence. Specifically, such methods are used for the measurement of coatings of tin on steel (tinplate), or the coating of zinc on steel (galvanized steel). Other coatings may comprise metals such as nickel, chromium, aluminum, or brass, as well as many other metals. The substrate can be of steel, aluminum, brass or other metal. The range of coating density can vary considerably but are generally between a few grams per square meter up to several hundred grammes per square meter.
Known methods, such as disclosed in U.S. Pat. Nos. 2,711,480 and 2,926,257 to Friedman, and U.S. Pat. No. 2,925,497 to Bessen for example, are employed to measure the thickness of such a metal coating on another metal. The are generally based on either of the two following phenomena:
A. The fluorescence of the substrate material when illuminated by certain radiation. The thickness of the coating is then determined by its absorbing action on the fluorescence beam emitted by the substrate. PA1 B. The fluorescence of the coating itself which is approximately proportional to the coating itself.
Sources of radiation used may be radioisotopes producing gamma rays of fixed energy or X-ray sources which produce a spectrum of radiation with the energy of each particle being at any level up to that represented in kiloelectron-volts by the kilovolt difference of potential across the X-ray tube.
In U.S. Pat. Nos. 2,711,480 and 2,926,257, Friedman describes fluorescence gauges to measure coatings on a substrate where he excludes the case of the coating and the substrate containing the same element.
In U.S. Pat. No. 2,711,480 Friedman describes the use of a monochromator (energy dispersive crystal), using the Bragg principle, between the fluorescence radiation and the detector, to discriminate between the fluorescence of the substrate and the coating. In U.S. Pat. No. 2,925,497, Beggen describes the use of a monochromator between the source of radiation and the coating to select preferentially the excitation energy from the radiation source. In both cases the use of a monochromator reduces significantly the radiation intensities involved in the measurement and accurate results can be obtained only if the measurements are made over several seconds which makes it difficult to use on a fast moving production line.
The problem is more difficult when the coating consists of two materials, and is even more difficult if the coating and the substrate contain the same element. In U.S. Pat. No. 4,764,945, Abe Tahidaro, (1988), addresses that difficult case by combining fluorescence measurements with Bragg diffraction measurements for a selected energy; (chrome Koc), performed at different angles, for the case of galvanneal products.
Again the main problem with this method is that the signals involved with the Bragg diffraction are extremely weak, and the time to make accurate measurements are very long especially for fast production lines. Also the apparatus is extremely complicated and mechanically slow requiring a detector rotating on an arm at accurate angles to measure the Bragg characteristic lines created by the crystalline structure of the material measured or the monochromator.